1. Technical Field of the Invention
The present invention relates to a combination die assembly which comprises a female die and a male die combined with the female die and is adapted for use in extruding small-, medium- or large-sized articles such as the multi-bored flat tubes in a heat exchanger made of aluminum or its alloy, and also the present invention relates to a method of extruding such articles by using the combination die assembly.
2. Prior Art
FIG. 12 shows an example of aluminum tubes which constitute a heat exchanger employed for instance in the air conditioning system. Among the various methods of manufacturing such a tube 1, the extrusion method is advantageous in that a high pressure resistance can be enhanced to the tube.
The die assembly, for example the so-called "porthole" die, used to extrude the tubes comprises in general a male die and a female die. The male die forms a hollow space extending through the tube 1, whereas the female die forms a periphery of said tube.
Since the multi-bored hollow and flat tubes 1 for the heat exchanger have a width "B" of 10-20 mm and a height "H" of 3-7 mm, the die assembly must be of a high precision despite its small size. Therefore, efforts have been made for a higher mechanical strength of the die assembly by improving its material. It is required that the repeated replacement of a worn die assembly with a new one does not unreasonably raise the running cost, and also that such a small-sized die assembly can be manufactured easily not to raise said cost. For this purpose, an improved male die 52 was already proposed, which die comprises separable parts as shown in FIG. 14a and has been used in combination with a female die 51. The male die 52 is composed of: a core 54 having portions 53 to form the hollow regions through the tube; and a mold 55 which holds the core 54 in place. Similar improvement is also being made for a larger die assembly extruding the medium- or large-sized particles.
Various structures have been proposed to let the core 54 be surely and precisely set in the mold 55 of the composite male die in a combination die assembly.
According to the proposal illustrated in FIG. 14a, the male die 52 has such a core 54 that is generally flat but is formed with a pair of ears 60. Each ear 60 integrally protrudes from the outer end of the core 54 in the direction of its thickness. A core-holding aperture 58 is formed through the mold 55 so as to receive the core 54, wherein its ears 60 rest on and are thus supported by shoulders which are formed in the aperture 58.
In a further proposal shown in FIG. 14b, the core 54 in the male die 52 has ears 57 integrally extending from the opposite sides at the outer end of the core. Those ears 57 are likewise supported by similar shoulders disposed in the core-holding aperture 58 of the mold 55.
In a still further proposal shown in FIG. 14c, the core 54 has opposite sides which are tapered to reduce the width of said core towards its inner end. The core receiving aperture 58 is also tapered at its sides so as to tightly engage with the tapered sides of the core.
As will be seen in the structure shown in FIG. 14a, the core 54 having the ears 60 protruding in the direction of thickness will render the core stereoscopic and somewhat complicated in shape. Consequently, it is not easy to manufacture the core at a reasonable cost. There is a likelihood that stress is concentrated at a corner between the core and either or both ears 60 during the extrusion process. Such a concentrated stress will produce a crack in the corner region, thus impairing the reliability in mechanical strength of the combination die assembly.
Although the core 54 shown in FIG. 14b can be more easily manufactured from a flat plate, there is a higher possibility that cracks are produced in the root portion of the ears 57, thereby breaking same early. This will result not only in a much lower reliability of the die assembly but also necessitate many replacements therefor.
In a further proposal shown in FIG. 14c, an extremely high precision must be ensured for the tapered portions of the core 54 which fits in the aperture formed through the mold 55, in order that the core can be positioned as accurate as possible relative to the mold with respect to the direction of extrusion. This requirement raises the manufacture cost of the combination die assembly.